If Python lags behind and there are multiple mouse release events processed after each other without mouse press events, then it would crash. No longer.
It should be a property of the node you're drawing on. Also this looks nicer because the selection outline won't avoid the custom support then. It looks much more like the actual overhang colour.
It's now so fast that it's not really necessary any more. By computing the support in the main thread, we prevent redrawing and so the red part won't disappear for a while while it is processing. It'll just stay there until processing is complete. This makes the support drawing feel much smoother.